This invention is an improvement on a crash sensing switch that is in commercial production which has a ball that moves in a sealed tube to bridge a pair of electrical contacts. During a crash the inertia of the ball causes it to move toward the contacts. For the ball to reach the contacts air must flow around the ball between the ball and the tube. Viscosity of the air in combination with inertia of the air causes a pressure differential that opposes ball movement through the tube. It is believed that in most vehicle crashes the viscous contribution to the pressure differential predominates over the inertial contribution. Viscous flow is proportional to the pressure differential which is proportional to the product of the mass of the ball and the deceleration of the crash sensing switch. The proportionality of viscous flow to deceleration and the requirement that the ball move a predetermined distance to bridge the contacts causes the current production crash sensing switch to be an acceleration integrator that completes the firing circuit upon a predetermined vehicular velocity change. A permanent magnet in the commercial production crash sensing switch provides a bias force that urges the ball toward a normal resting position away from the electrical contacts. The bias force also causes the vehicular velocity change required for switch closure to increase with the duration of the crash.
The tube and ball of the commercial production crash sensing switch are made of stainless steels having different temperature expansion coefficients to provide compensation for the variation of air viscosity with temperature. The differential thermal expansion causes the gap between the ball and tube to change with temperature and reduce the effect of variation of air viscosity over the range of operating temperatures. An elastomeric seal in the commercial production crash sensing switch reduces transmission of cross axis vibrations to the ball and tube.
This commercial production crash sensing switch is expensive to manufacture and one reason for the high cost is the high precision required of the ball and the tube. Another reason for the high cost is that the tube material is difficult to work. Further, connecting the electrical contacts, lead wires and a diagnostic resistor by soldering creates contamination that cannot be tolerated near the ball and tube. Therefore, additional components and processing steps are required to protect the ball and tube during manufacturing. This commercial production crash sensing switch is particularly sensitive to contamination because small particles can wedge between the ball and the tube and interfere with movement of the ball.
When a vehicle going at high speed strikes a rigid obstacle the very high deceleration causes high air velocity in the commercial production crash sensing switch. Overcoming air inertia reduces the pressure available to overcome viscous resistance to air flow. Furthermore, the pressure required to overcome inertia is independent of temperature because it does not depend on viscosity resulting in excessive temperature compensation. Also, when the flow is inertial the crash sensing switch is not a velocity integrator because inertially damped air flow does not increase linearly with pressure.
In another known crash sensing switch having a ball and a tube a spring provides the bias force. A permanent magnet has the advantage of simplicity but the cost is significant and its large size increases the size of the crash sensing switch. Another difference is that the magnetic force decreases as the ball moves toward the electrical contacts whereas the spring force increases. In most designs the bias force is small therefore sensors with spring bias and sensors with magnetic bias perform similarly.
U.S. Pat. No. 4,932,260 issued Jun. 12, 1990 to Peter Norton for "Crash Sensing Switch With Suspended Mass" describes a crash sensing switch having a suspended mass in which air ducts conduct the air displaced by the movement of the sensing mass and in which varying the normal resting position of the armature provides compensation for variation of air viscosity with temperature by varying the armature travel according to the temperature.
Copending application Ser. No. 183,134 filed Jan. 18, 1994 for "Compact Crash Sensing Switch With Air Channels and Diagnostic System" describes a crash sensing switch having a much smaller ball than current production crash sensing switches sealingly movable in a tube and in which air ducts conduct most of the air displaced by the movement of the ball and in which a semiconductor device completes the firing circuit. This crash sensing switch also uses the semiconductor circuit to provide diagnostic capabilities. The semiconductor switch and diagnostic features of that invention are applicable to the present invention.
For certain applications, for example when used as a passenger compartment crash sensing switch, a viscously damped crash sensing switch is designed to close at a much lower velocity change at high decelerations than at lower decelerations. This is accomplished by making the bias force larger relative to the viscous damping forces.
It has recently become common to place an electronic crash sensing switch in or near the passenger compartment of a vehicle. These crash sensing switches are known as "single point crash sensors". This design is advantageous because it can eliminate the wiring and assembly time required when the crash sensing switch is located in the forward part of the vehicle. However, in certain vehicles it has not been possible to identify all crashes requiring air bags from information received at a single point in the passenger compartment. For those vehicles one solution is to place an auxiliary sensor in the forward part of the vehicle and connect it to the "single point crash sensor" to provide additional information to enable timely detection of all crashes. Forward auxiliary sensors or "auxiliary discriminating sensors" are known by the acronym "ADS". An ADS needs to conduct only a few milliamperes to initiate operation of an electronic circuit and it needs only to close, it does not need to remain closed for a significant period of time.
The crash sensing system of a vehicle must initiate deployment of the occupant protection systems some time before the protection is required. The crash sensing switch is typically required to initiate deployment of air bags 30 milliseconds before a free body in the passenger compartment moves six inches (150 mm.) forward from its pre crash position. The basis for this requirement is that an air bag should be deployed before the occupant has moved six inches (150 mm.) and deployment of an air bag typically takes 30 milliseconds.
Pyrotechnic devices provide the gas to inflate air bags. Several well-known inflators use the decomposition of sodium azide to generate gas for inflating the air bag. Another design, known as a hybrid design, uses a combustible material to heat stored argon gas for filling the air bag.
The rate at which inflators inflate air bags depends on the temperature of the inflator. At higher temperatures inflators produce a greater volume of gas and produce it more rapidly than at lower temperatures. This is true of both inflators based on decomposition of sodium azide and hybrid inflators. At the lowest temperatures in the operating range deployment may be delayed by ten milliseconds or more relative to a nominal deployment. At the highest temperatures in the operating range deployment may be accelerated by ten milliseconds or more. In prior occupant protection systems this effect has been ignored. One reason it has been ignored is because the temperature of the inflator has been assumed to be independent of ambient temperature because it is located in the passenger compartment where temperatures are controlled to make the occupants comfortable. However, the air bag and other apparatus surrounding the inflator thermally insulate it from the passenger compartment and there is a delay between the time the passenger compartment air reaches a controlled temperature and the time the inflator temperature approaches the passenger compartment air temperature. Consequently, on cold days inflators are colder on average than on hot days and air bag deployment occurs more rapidly on average on hot days than on cold days.
Consider, for example, an air damped crash sensing switch without compensation for the variation of air viscosity with temperature and designed to close at a velocity change of nine miles per hour at a nominal temperature. At a very cold temperature it would close at a velocity change of about seven miles per hour therefore it might close a few milliseconds sooner than if it had temperature compensation. Closing sooner may be desirable because it might compensate for slower deployment of the air bag at low temperatures. The advantage of earlier initiation when deployment is wanted might justify a small number of undesired deployments during low speed crashes at low temperatures.
Considering further the example of an uncompensated nine mile per hour air damped crash sensing switch. At a very high temperature it might close at a velocity change of eleven miles per hour therefore closing a few milliseconds later than if it had temperature compensation and it would not close during low velocity crashes having velocity changes between nine and eleven miles per hour. Neither of these consequences is desired.
It follows that a crash sensing switch with compensation for variations of air viscosity with temperature at temperatures above a nominal temperature but without compensation at temperatures below a nominal temperature may be desired.
Thermostat metals are sheets made of layers of metals having different thermal expansion coefficients that flex as the temperature changes. These materials are well known and have been used for many years in such as home thermostats. A wide selection of materials is presently available commercially from several suppliers. One of these suppliers is Texas Instruments.
Polytetrafluoroethylene (abbreviated PTFE) is sold under various trade names, one of which is "Teflon". Polytetrafluoroethylene and materials containing it can be inexpensively coated on metals to provide a low coefficient of friction.
Certain plastics, one of which is polyphenylene sulfide, combined with filaments of fiberglass can be molded to make objects having accurate linear dimensions. Polyphenylene sulfide also offers superior resistance to moisture absorption and moisture vapor permeation, good mechanical strength, and dimensional stability over a wide temperature range.
Crash sensing switches having ducts for viscously conducting air displaced by movement of sensing masses are described In U.S. Pat. No. 4,932,260 and in the copending application Ser. No. 183,134 referred to hereinabove.
One advantage of using air ducts to meter air flow instead of using the space between a ball and a tube as is done in the hereinabove described commercial production crash sensing switches is that the greater length and four sides of the ducts compared with the much shorter effective length and two sides of the gap between the ball and the tube enable the ducts to have larger width dimensions than the gap between the ball and the tube which reduces manufacturing cost. Another advantage is that the resistance of the air ducts to air flow can be adjusted during manufacture. One way to adjust the resistance of the ducts to air flow is by opening or plugging some of the ducts. Where the ducts result from mating a fluted surface and a smooth surface, another way to adjust the resistance of the ducts to air flow is by adjusting the pressure between the lands of the fluted surface and the smooth surface.
Another advantage of using air ducts to meter air flow is that air ducts provide design flexibility not possible in a crash sensing switch of the commercial production ball in tube design. As described hereinabove, air inertia affects air flow at high decelerations. In the aforementioned commercial production crash sensing switches the accelerations at which air inertia becomes significant are completely determined by the mass of the ball and the specified calibration. Ducts enable control of the vehicle deceleration at which air flow becomes significantly affected by inertia. A larger number of smaller ducts provide lower air velocity and smaller inertial effects than fewer larger ducts at the same pressure. Therefore, an advantage is that the number of ducts may be varied to adjust the degree to which the air flow is affected by inertia. Even greater flexibility results because in a single crash sensing switch different ducts may be made with different width dimensions thereby causing different inertial effects in different ducts.
Before crash sensing switches based on flow of air in ducts were made and tested it was anticipated that the air flow would be turbulent in larger ducts at higher pressures because the Reynolds number would be large. The Reynolds number is the product of the duct width, the air density, the velocity of air flow and the reciprocal of the viscosity. It commonly stated that flow is likely to be turbulent at Reynolds numbers greater than 2000. However, our experience is that in ducts as large as 0.5 millimeter square and 11 millimeters long and at pressures as high as twenty pounds per square inch the flow rate through the duct is accurately represented by the equations of laminar flow.
A general object of this invention is to provide a crash sensing switch for automotive vehicles which also overcomes certain disadvantages of the prior art.